At the heart of the BDS is the ORBTRAC 300 Fleet Management System, developed by Orbital Transportation Management Systems, of Virginia. Basically a sophisticated Automatic Vehicle Location System (AVLS), it uses data from the global positioning system (GPS) and GIS spatial analysis to provide dispatchers with realtime bus locations, operational data and security communications. The main elements of the ORBTRAC are the GPS satellites, a differential correction station and onboard sensors. At the command center, nine UNIX workstations with two color monitors display bus locations and movements and a prioritized list of events requiring action by the dispatcher or driver.
A GPS receiver aboard the bus determines its position by triangulating on signals received from three or more satellites. Since the signals are affected by atmospheric conditions such as rain and snow, and by nearby buildings and trees, as well as by selective availability, a location derived from raw GPS data can be off by as much as 100 meters. To correct such errors, a computer interfaced with a GPS receiver at a stationary site continually compares its known location with the one reported by its receiver. The computer calculates the difference and broadcasts corrections, which are picked up by the AVL aboard the bus and applied to its raw GPS location data, correcting the location of the bus to within 10 meters -- about one bus length.
The onboard computer automatically relays its corrected location to the command center, along with data from onboard sensors that monitor passenger count, door motion, wheelchair lift and other bus operations. The onboard computer also provides two-way voice and digital communication between drivers and dispatchers. If the bus is off-route, behind or ahead of schedule, the computer automatically sends that information to the command center. At the end of each day, the computer uploads the day's recorded schedule and route data to the command center, where the information is automatically summarized and stored for later analysis. On newer buses, computers automatically change the bus-destination sign, and count passengers as they board and alight.
Command Center
The data from the buses automatically uploaded to the command center are summarized into tables for the monthly and quarterly reports and stored in a database for three months. Tri-Met analyzes the summaries for flagged irregularities, ridership patterns and complaints, driver performance, maintenance reports and sources of data inaccuracies. The objective is to assess the effectiveness of the overall system and adjust schedules to meet constantly changing priorities for public transit.
The Role of GIS
According to Tri-Met Coordinator for Service and Performance Analysis Steve Callas, the BDS uses a ESRI basemap maintained by the Regional Metropolitan Government. Tri-Met's GIS department provides the map overlays -- routes, stops, shelters, etc. -- and updates the PCMCIA card aboard the bus. "We call that the crunch process; every time we have a schedule change, we run that process, and the cards are automatically updated with the correct schedule."
About 40 percent of Tri-Met buses now have automatic passenger counters integrated with the AVL system to record the on-and-offs at individual stops. "We're going to put that data into a database linked to the GIS so we'll be able to summarize the boardings and alightings for each stop in the entire system. We'll analyze the derived data to determine bus stops that need shelters. We'll also use the data for planning; for example, we're looking at ways to speed up our buses. The stops on most of our routes are two blocks apart. The data will show the stops that get the heaviest and lightest use, perhaps allow us to consolidate some stops and speed up the buses. We are continually looking at ways to make the bus system more efficient, reduce the number of buses during off-peak times, and give higher priority to other routes with greater passenger traffic."
Security
The BDS' communication capabilities have also improved security on Tri-Met buses. If a driver sends a silent alarm, the system immediately opens a covert microphone aboard the bus, enabling the dispatcher to hear what is happening and determine an appropriate response. Turner said drivers are trained to give voice clues as to the nature of the emergency. "For example, if a weapon is involved, they are supposed to make comments like, 'If you put that gun away, it won't go off by accident.' That's all the dispatcher has to hear to contact the police and give the exact location of the bus. On the other hand, if the dispatcher hears only normal conversation, the police are still notified that an alarm has been received. The driver may have bumped the button by accident, but we call the police either way.
"Before we had this system, we would get an emergency message but the driver would often forget to give a location," Turner said. "Also, we've had cases where the driver started moving the bus to get a passenger having a heart attack closer to the hospital. As a result, the ambulance couldn't find the bus. With BDS, we know exactly where the bus is, even if it moves. The system has definitely improved safety and security."
Planned Expansion
In the two years BDS has been in operation, a period Turner describes as "phase one," Tri-Met has focused on improving data accuracy and distribution, and planning expanded system capabilities. In phase two, scheduled for next year, the agency will display realtime arrival information at major bus stops, increase the ability of the system to distinguish different types of traffic delays and interconnect the BDS with the Regional Traffic Management System to provide video images of busy intersections and freeways. The ability to view traffic conditions will assist dispatchers in anticipating detours before buses are blocked by congestion. Tri-Met also plans to develop data-warehousing and -retrieval capability, and install mobile data terminals in the cars of field supervisors, enabling them to handle certain situations directly.
Before moving to phase two, however, Turner said Tri-Met is focusing on improving the accuracy of the data being put into the DBS. "The system itself actually works very well and very reliably -- when it has good data. Right now, we need to get a better match between the data we're feeding to the bus and the current schedule and route configuration the bus is following; sometimes things change on the street and we don't get the information updated for a week or so. "A transit system is not a static or constant operation; bus stops are moved regularly, routes are changed for various reasons, because of construction or to serve a larger group of people -- all of those changes have to be timely and accurately reflected in the data before buses can navigate the system and send the right information back to the command center.
"We have to get to the point where we can provide accurate bus-arrival times for the customer," Turner added. "If the bus isn't there when it's supposed to be, the other amenities don't help. Our next step is to provide reliable, realtime information so people can depend on it."
In phase three, Tri-Met plans to interface the BDS with the traffic lights at about 250 intersections throughout the city. The extension will automatically trigger signal lights to favor a bus if it is running late. "We don't do this lightly," Turner stressed. "We're making sure that we have a system robust enough to layer something like this on top of it. To work properly, the system must accurately predict whether the bus is early or late; it wouldn't do to hold a green light for a bus running ahead of schedule. Also, if two buses are approaching an intersection from different directions, one two minutes late and the other seven minutes, the signal controller should favor the later bus. The city is currently developing software that will determine how much priority to give a bus and which bus would receive it. We think the bus system eventually will be able to communicate how late it is to the traffic controller."
Investment
According to Turner, the total cost of the DBS project to date is $6 million, 80 percent of which was underwritten by the Federal Transportation Administration. Tri-Met provided the remaining 20 percent.
Over the last fiscal year, Tri-Met records show a significant increase in ridership. However, the agency is not attributing the rise solely to the new bus dispatch system. Several other factors are involved, including growth in the region.
"We currently have Portland State University doing an evaluation of the bus dispatch system. One of the things they found is that buses are running more on time than they were before," Turner said. "We do know that the DBS has helped us improve the stability of our operations and provide a more dependable service with greater security. Ridership is increasing, people are getting out of their cars, that's the real goal."
--------------------------------------------------------------------------------
Bill McGarigle is a writer, specializing in communications and information technology. He is based in Santa Cruz, Calif. E-mail
